Fluidized bed technologies have been applied to a type of hydrocarbon processing known as “coking”. In a commercial coking process a hydrocarbon feed is reacted at temperatures greater than approximately 350° C., and typically greater than 430° C., but typically less than 580° C. The targeted chemical species of the coking process reside for the most part in the “pitch” fraction of the feed, typically defined as the fraction of the oil that boils above 524° C., based on standard industry test methods. A number of fluid bed coking reactors have appeared in the patent literature since the 1940s, an example of which is disclosed in U.S. Pat. No. 2,895,904. The term “Fluid Coking” has become synonymous with the coking reactor described in this patent.
Another example of a conventional Fluid Coking reactor 15 having a fluidized bed 23 is shown in FIG. 1 (PRIOR ART). In the Fluid Coking process, hot solid particles enter the reactor 15 in a freeboard region 19, above the surface of the fluid bed 23 and are fluidized by fluidization gas. Solid particle withdrawal occurs at the bottom of the reactor 15. Feed is sprayed in the liquid phase into the fluid bed 23 at several different elevations 20 where it coats a portion of the fluidized solid particles. The nature of the solids mixing in the fluidized bed leads to the condition that solid particles within the fluidized bed is generally well mixed.
In the conventional Fluid Coking reactor shown in FIG. 1, a fraction of the feed consists of a liquid phase pitch that is distributed onto a fluidized bed of heated coke solid particles with the solid particles providing the thermal energy for the cracking reactions. The cracking reactions generate a solid hydrocarbon byproduct (“coke”) that is deposited onto the solid particles that were initially coated with liquid-phase pitch. The surface area provided by the fluidized solid particles results in a relatively high rate of heat transfer for these reactors. The Fluid Coking process is continuous, with solid particles being added and withdrawn at the same rate. After withdrawal the solid particles are heated up before being reintroduced back into the reactor. In addition, since coke is deposited onto the fluidized solids the solids inventory increases, and an equivalent amount of the solids must be purged in order to maintain steady state conditions within the reactor.
An operational challenge that exists with fluidized beds is to maintain fluidized conditions. When a bed “defluidizes” the drag force imparted by the movement of the gas relative to the solid particles is no longer able to support the weight of the solid particles. The bed then “slumps”, and intimate contact between the solid particles is re-established as the bed is no longer fluidized. A bed that is defluidized is said to be a “packed bed” of solids. Defluidization of a fluid bed during operation constitutes a serious operational challenge, since loss of bed fluidity results in a system that behaves in a manner that is inconsistent with a continuous fluid.
For a petroleum oil application in which the fluidized solid particles provide the energy required to convert a liquid hydrocarbon feed into lower boiling products and a condensed coke byproduct, the situation only worsens following a defluidization event. Any liquid present in the system at the time of the defluidization incident will continue to react. The coke formed will bridge the adjacent solids together, essentially cementing the entire bed together as a single cohesive unit. This problem is magnified if fresh feed addition is continued after the defluidization event. The end result is that the processing unit has to be shut down for maintenance, which requires the solid mass to be cut out of the reactor using water lasers, or other mechanical means. This activity is taken at considerable expense, with implications both upstream and downstream in the refinery.
The mechanism by which defluidization is initiated by the agglomeration of wet particles is of particular importance in a fluidized bed coking process. When a fluidized bed coking reactor defluidizes due to the introduction of too much liquid feed, the bed is said to have “bogged”, and the processes leading to the bogged bed is referred to as “bogging”. British patent 759,720 discloses operational guidelines for feeding a Fluid Coking process for converting a heavy hydrocarbon feed to lower boiling products, and in particular, defines a maximum feed rate below which defluidization by bogging will be avoided. In this patent, a Fluid Coking process is disclosed wherein hot fluidized solids are fed continuously into a fluid bed coking reactor, with cool solids withdrawn at the same rate. As described in the patent, the bulk of the data were obtained in a laboratory-scale fluidized bed unit in which the fluidized solids were not added or withdrawn from the reactor; this mode of operation is referred to in the basic chemical engineering literature as a “fed batch” reactor. Data from the fed batch reactor were used to empirically formulate a mathematical relationship used to calculate the maximum possible feed rate at which fluidized conditions could be maintained. The inputs to the model were: the reactor temperature, and the amount of coke forming material in the fresh feed, as determined by the standard “Conradson Carbon Number (CCR)” test. It is well known within the industry that the “Micro Carbon Residue (MCR)” test or equivalent could be applied effectively in place of the CCR. An empirical factor was included that captures the impact of scale-up, the efficiency of feed distribution on the particles, the characteristics of the fluidized solids, and the fluidization gas rate.
While British patent 759,720 discloses a method to feed a Fluid Coking process, the data accumulated for the model were acquired using a fed batch reactor. A fed batch reactor configuration substantially differs from the Fluid Coking process, the most significant difference being no circulation of solids in a fed batch reactor. Therefore, it is unclear whether it is accurate to base the prediction of defluidization in a fluid bed coking reactor from data obtained from a fed batch reactor. Further, British patent 759,720 does not provide any insight into how to efficiently operate a fluidized bed reactor exhibiting mixing characteristics that are not well mixed with respect to the fluidized solids. In particular, it is not clear how applicable the method disclosed in British patent 759,720 is for feeding a fluidized bed reactor with primarily plug flow characteristics, such as a cross-flow fluidized bed reactor as disclosed in Applicant's own PCT publication no. WO 2005/040310.